The present invention relates generally to electrical fuse circuit designs, and more particularly to designing a self test and repair module for the electrical fuse circuit.
Electrical fuse is a convenient logic nonvolatile memory for permanently holding information such as “chip-ID”, etc. A typical implementation is a laser-fuse, where laser energy is used to program the fuse by evaporating metal or polysilicon links and the resulting resistance change is sensed using a latch. However, pitches of the laser-fuse device are not scalable below the wavelength of the laser beam, typically 1.06 um, thus the laser-fuse is not suitable for deep submicron technologies.
To overcome the laser-fuse's pitch limitation, an electrical fuse, typically made of silicided polysilicon, uses electrical current instead to program. When programming the electrical fuse, a high current density, typically 600 mA/um2 for the silicided polysilicon, is applied to the electrical fuse link for a certain period of time. The resistance of the electrical fuse will rise due to the electrical stress in its fuse link. A few micro seconds of stress may be a long enough time to cause a discernable resistance change, ideally more than 1 Kohm, in the electrical fuse.
Since electrical fuse exhibits no visible signs of change after being programmed. Testing of the electrical fuse under program can only be performed electrically. A conventional method is to test the electrical fuse on an automatic test equipment (ATE). However, spending test time on the ATE will increase the cost of manufacturing the integrated circuit (IC) that contains the electrical fuse. Besides, the ATE can not perform any repair if a faulty electrical fuse is encountered. Such an IC chip will be discarded for the faulty electrical fuse even if the main circuit performs perfectly. This lack of reparability will affect product yield and increase the overall cost of manufacturing the IC chip.
As such, what is desired is an electrical fuse circuit that can be easily tested and repaired after being programmed.